1. Field of Invention
The present invention relates to a chamber for measuring the pressure inside a pipe, in particular in an extra-corporeal circuit.
2. Description of the Prior Art
During therapeutic treatment requiring extra-corporeal circulation, as for example in the case of hemodialysis, the pressure inside the circuit must be monitored. For this purpose, the machine used for the therapeutic treatment usually comprises suitably designed sensors. It is obviously necessary to prevent the fluids contained in the circuit from contaminating these sensors which are intended to be used repeatedly. On the other hand, the extra-corporeal circulation is performed in a disposable circuit.
For this purpose it is known to provide the circuit with at least one pressure chamber, usually called a “pressure dome”, suitable for creating an interface between the circuit and the pressure sensor. The pressure dome usually comprises a housing with an inlet and outlet which are respectively connected to the circuit. Finally an elastomer membrane closes one side of the chamber and is formed so as to be able to make contact with a pressure sensor. The elastomer membrane is highly resilient so that it is able to transmit to the sensor the pressure present inside the circuit and the associated variations. A pressure dome of this type is schematically shown in FIG. 1 and is described in detail in the U.S. Pat. No. 7,603,907.
These pressure domes, although widely used are however not defect-free.
A first defect consists in the fact that air may infiltrate between the membrane and the sensor. This phenomenon may occur, for example, when the sensor is joined to the pressure dome. During positioning of the sensor it is possible in fact for an air pocket to remain trapped between the two surfaces which instead should remain in direct contact. In such a case the sensor is no loner able to measure adequately the pressure of the chamber nor provide a reliable response with regard to variations thereof.
This problem is further exacerbated should the circuit have internally a negative pressure, a pressure lower than atmospheric pressure. In such a case, which typically occurs upstream of the pumps located along the circuit, the membrane assumes a concave form, i.e. is “sucked” towards the inside of the pressure dome and worsen the precision of measurement.
Furthermore there exists the problem of ageing of the elastomer from which the membrane is made. This ageing results in the loss of flatness of the membrane. It is clear that a membrane which has lost its elasticity and its flatness may easily result in the formation of air pockets between the pressure sensor and the membrane itself. It should be remembered in this connection that the pressure domes and associated membranes are generally intended to have a shelf life of several years from time of production to actual use. This shelf life, which is quite reasonable from a logistical point of view, risks a loss of its design characteristics.
Another problem of the pressure dome of the known type is instead associated with the technology which is currently used to manufacture it. The main body of the chamber is made, in a manner known per se, by means of molding of a polymer which is sufficiently rigid and suitable for contact with the physiological fluids. The elastomer membrane, instead, is made by means of two-component injection molding which is used to produce, in addition to the actual elastomer membrane, also a fixing ring made with a rigid polymer, for example such as that used for the main body. The elastomer membrane and the associated ring thus form a single part made of two different materials. They are then joined to the main body, for example by means of a screw/female-thread, snap-engaging, interference or similar connection.
The manufacture of the pressure dome of the known type, which comprises a two-component injection molding operation, therefore requires the use of molds with movable parts, the manufacture and use of which are somewhat complex. Moreover these movable-part moulds require an initial outlay which is distinctly greater than that of ordinary molds.